The Thermo-couple

With the application of the thermo-couple, the measurement of
temperatures, between, say, 700 and 2,500 deg.F., was made more simple
and precise. The theory of the thermo-couple is simple; it is that
if two bars, rods, or wires of different metals are joined together
at their ends, when heated so that one junction is hotter than the
other, an electromotive force is set up through the metals, which
will increase with the increase of the difference of temperature
between the two junctions. This electromotive force, or voltage, may
be measured, and, from a chart previously prepared, the temperature
determined. In most pyrometers, of course, the temperatures are
inscribed directly on the voltmeter, but the fact remains that
it is the voltage of a small electric current, and not heat, that
is actually measured.

There are two common types of thermo-couples, the first making use
of common, inexpensive metals, such as iron wire and nichrome wire.
This is the so-called base metal couple. The other is composed of
expensive metals such as platinum wire, and a wire of an alloy of
platinum with 10 per cent of rhodium or iridium. This is called
the rare metal couple, and because its component metals are less
affected by heat, it lasts longer, and varies less than the base
metal couple.

The cold junction of a thermo-couple may be connected by means
of copper wires to the voltmeter, although in some installations
of base metal couples, the wires forming the couple are themselves
extended to the voltmeter, making copper connections unnecessary.
From the foregoing, it may be seen that accurately to measure the
temperature of the hot end of a thermo-couple, we must know the
temperature of the cold end, as it is the difference in the
temperatures that determines the voltmeter readings. This is absolutely
essential for precision, and its importance cannot be over-emphasized.

When pyrometers are used in daily operation, they should be checked
or calibrated two or three times a month, or even every week. Where
there are many in use, it is good practice to have a master pyrometer
of a rare metal couple, which is used only for checking up the
others. The master pyrometer, after calibrating against the melting
points of various substances, will have a calibration chart which
should be used in the checking operation.

It is customary now to send a rare metal couple to the Bureau of
Standards at Washington, where it is very carefully calibrated
for a nominal charge, and returned with the voltmeter readings
of a series of temperatures covering practically the whole range
of the couple. This couple is then used only for checking those
in daily use.

Pyrometer couples are more or less expensive, and should be cared
far when in use. The wires of the couple should be insulated from
each other by fireclay leads or tubes, and it is well to encase them
in a fireclay, porcelain, or quartz tube to keep out the furnace
gases, which in time destroy the hot junction. This tube of fireclay,
or porcelain, etc., should be protected against breakage by an
iron or nichrome tube, plugged or welded at the hot end. These
simple precautions will prolong the life of a couple and maintain
its precision longer.

Sometimes erroneous temperatures are recorded because the cold
end of the couple is too near the furnace and gets hot. This always
causes a temperature reading lower than the actual, and should be
guarded against. It is well to keep the cold end cool with water,
a wet cloth, or by placing it where coal air will circulate around
it. Best of all, is to have the cold junction in a box, together
with a thermometer, so that its temperature may definitely be known.
If this temperature should rise 20 deg.F. on a hot day, a correction of
20 deg.F. should be added to the pyrometer reading, and so on. In the
most up-to-date installations, this cold junction compensation is
taken care of automatically, a fact which indicates its importance.

Optical pyrometers are often used where it is impracticable to
use the thermo-couple, either because the temperature is so high
that it would destroy the couple, or the heat to be measured is
inaccessible to the couple of ordinary length. The temperatures of
slag or metal in furnaces or running through tap-holes or troughs
are often measured with optical pyrometers.

In one type of optical pyrometer, the observer focuses it on the
metal or slag and moves an adjustable dial or gage so as to get
an exact comparison between the color of the heat measured with
the calor of a lamp or screen in the pyrometer itself. This, of
course, requires practice, and judgment, and brings in the personal
equation. With care, however, very reliable temperature measurements
may be made. The temperatures of rails, as they leave the finishing
pass of a rolling mill, are measured in this way.

Another type of optical pyrometer is focused on the body, the
temperature of which is to be measured. The rays converge in the
telescope on metal cells, heating them, and thereby generating a
small electric current, the voltage of which is read an a calibrated
voltmeter similar to that used with the thermo-couple. The best
precision is obtained when an optical pyrometer is used each time
under similar conditions of light and the same observer.

Where it is impracticable to use either thermo-couples or optical
pyrometers, sentinels may be used. There are small cones or cylinders
made of salts or other substances of known melting points and covering
a wide range of temperatures.

If six of these sentinels, melting respectively at 1,300 deg., 1,350 deg.,
1,400 deg., 1,450 deg., 1,500 deg., and 1,550 deg.F., were placed in a row in a
furnace, together with a piece of steel to be treated, and the
whole heated up uniformly, the sentinels would melt one by one and
the observer, by watching them through an opening in the furnace,
could tell when his furnace is at say 1,500 deg. or between 1,500 deg. and
1,550 deg., and regulate the heat accordingly.

A very accurate type of pyrometer, but one not so commonly used as
those previously described, is the resistance pyrometer. In this
type, the temperature is determined by measuring the resistance to an
electric current of a wire which is at the heat to be measured. This
wire is usually of platinum, wound around a quartz tube, the whole
being placed in the furnace. When the wire is at the temperature of
the furnace, it is connected by wires with a Wheatstone Bridge, a
delicate device for measuring electrical resistance, and an electric
current is passed through the wire. This current is balanced by
switching in resistances in the Wheatstone Bridge, until a delicate
electrical device shows that no current is flowing. The resistance
of the platinum wire at the heat to be measured is thus determined
on the Bridge, and the temperature read off on a calibration
chart, which shows the resistance at various temperatures.

These are the common methods used to-day for measuring temperatures,
but whatever method is used, the observer should bear in mind that
the greatest precision is obtained, and hence the highest efficiency,
by keeping the apparatus in good working order, making sure that
conditions are the same each time, and calibrating or checking
against a standard at regular intervals.